1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same, and to a thin-film magnetic head, a head assembly and a magnetic disk drive each of which includes the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include GMR (giant magnetoresistive) elements utilizing a giant magnetoresistive effect, and TMR (tunneling magnetoresistive) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, GMR heads that employ spin-valve GMR elements have been mass-produced. Recently, to accommodate further improvements in areal recording density, developments have been pursued for read heads employing TMR elements.
A spin-valve GMR element typically includes a free layer, a pinned layer, a nonmagnetic conductive layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer having a direction of magnetization that changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer having a fixed direction of magnetization. The antiferromagnetic layer is a layer that fixes the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element. Such a structure is called a CIP (current-in-plane) structure. On the other hand, developments have been pursued for another type of GMR heads having a structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element. Such a structure is called a CPP (current-perpendicular-to-plane) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
Read heads that employ TMR elements mentioned above have the CPP structure, too. A TMR element typically includes a free layer, a pinned layer, a tunnel barrier layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer through which spin-conserved conduction electrons are capable of transmitting by the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer of the TMR element are the same as those of the spin-valve GMR element. As compared with the spin-valve GMR element, the TMR element is expected to provide a higher magnetoresistance change ratio (hereinafter referred to as an MR ratio), which is the ratio of magnetoresistance change with respect to the resistance.
JP 2003-008102A discloses a CPP-GMR element including: a pinned layer whose direction of magnetization is pinned; a free layer whose direction of magnetization changes in response to an external magnetic field; a nonmagnetic metal intermediate layer provided between the pinned layer and the free layer; and a resistance adjustment layer provided between the pinned layer and the free layer and made of a material containing conductive carriers not more than 1022/cm3. JP 2003-008102A discloses that the material of the resistance adjustment layer is preferably a semiconductor or a semimetal.
JP 2003-298143A discloses an MR element of the CPP structure including a pinned layer whose direction of magnetization is pinned, a free layer whose direction of magnetization changes in response to an external magnetic field, and an intermediate layer located between the pinned layer and the free layer, wherein the intermediate layer includes a first layer (an intermediate oxide layer) made of an oxide and having a region in which the resistance is relatively high and a region in which the resistance is relatively low, and wherein, when a sense current passes through the first layer, the sense current preferentially flows through the region in which the resistance is relatively low. JP 2003-298143A discloses that the sense current has an ohmic characteristic when passing through the first layer. Therefore, the MR element disclosed in this publication is not a TMR element but a CPP-GMR element. Such a CPP-GMR element is called a current-confined-path type CPP-GMR element, for example. JP 2003-298143A further discloses that the intermediate layer further includes a second layer (an interface adjusting intermediate layer) made of a nonmagnetic metal that is disposed between the first layer and the pinned layer, and between the first layer and the free layer.
JP 2005-086112A also discloses a current-confined-path type CPP-GMR element. This CPP-GMR element includes two nonmagnetic intermediate layers disposed between the free layer and the pinned layer, and a current control layer disposed between the two nonmagnetic intermediate layers. A conductive film made of, e.g., Cu, is used for each of the two nonmagnetic intermediate layers. The current control layer is composed mainly of an insulator that electrically insulates layers disposed on top and bottom of the current control layer from each other, and conductive materials that electrically connect the layers disposed on top and bottom are provided in such a manner as to be scattered in the insulator.
JP 2006-261306A also discloses a current-confined-path type CPP-GMR element. This CPP-GMR element includes an intermediate layer disposed between the pinned layer and the free layer. The intermediate layer includes an insulating film, and a columnar metal conduction portion formed within the insulating film. The CPP-GMR element further includes a compound layer formed between the metal conduction portion and one of the pinned layer and the free layer. The compound layer includes a compound having an ionic binding or covalent binding property. For example, a III-V semiconductor, a II-VI semiconductor or an oxide semiconductor is used as the material of the compound layer.
To use a TMR element for a read head, it is required that the TMR element be reduced in resistance. The reason for this will now be described. Improvements in both recording density and data transfer rate are required of a magnetic disk drive. Accordingly, it is required that the read head exhibit a good high frequency response. However, a TMR element with a high resistance would cause a high stray capacitance in the TMR element and a circuit connected thereto, thereby degrading the high frequency response of the read head. For this reason, it is required that the TMR element be reduced in resistance.
To reduce the resistance of the TMR element, it is typically effective to reduce the thickness of the tunnel barrier layer. However, an excessive reduction in the thickness of the tunnel barrier layer would cause a number of pinholes to develop in the tunnel barrier layer, resulting in a shorter service life of the TMR element. In addition to this, a magnetic coupling may also be established between the free layer and the pinned layer, resulting in deterioration of characteristics of the TMR element such as an increase in noise or a reduction in MR ratio. Here, noise that occurs in read heads is referred to as head noise. Head noise that occurs in a read head employing a TMR element includes shot noise which is a noise component that would not be generated in a read head employing a GMR element. For this reason, a read head employing a TMR element has a problem that it develops greater head noise.
On the other hand, a CPP-GMR element has a problem that it cannot provide a sufficiently high MR ratio. This is presumably because spin-polarized electrons are scattered at the interface between the nonmagnetic conductive layer and a magnetic layer or in the nonmagnetic conductive layer.
Additionally, a CPP-GMR element is low in resistance and is therefore small in resistance change amount. Accordingly, in order to obtain a higher read output with a CPP-GMR element, it is necessary to increase the voltage applied to the element. An increase in the voltage applied to the element would raise the following problem, however. In a CPP-GMR element, a current is fed in the direction perpendicular to the plane of each layer. This causes spin-polarized electrons to be injected from the free layer into the pinned layer or from the pinned layer into the free layer. These spin-polarized electrons generate a torque in the free layer or the pinned layer to rotate the magnetization thereof. In this application this torque is referred to as a spin torque. The spin torque is proportional to the current density. An increase in the voltage applied to the CPP-GMR element causes an increase in current density, thereby resulting in an increase in spin torque. An increase in spin torque results in a problem that the direction of magnetization of the pinned layer fluctuates.
JP 2003-008102A discloses that providing the resistance adjustment layer makes it possible to appropriately adjust the resistance of a CPP element and to thereby increase the resistance change amount so as to enhance the output. However, it is not always possible to increase the MR ratio simply by inserting the resistance adjustment layer between the pinned layer and the free layer. This is because, while the resistance of the MR element increases as the crystal structure of the resistance adjustment layer or a neighborhood thereof is disordered, the disorder of the crystal structure makes spin-polarized electrons scatter noticeably, and as a result, a reduction in MR ratio is caused by spin relaxation.
Current-confined-path type CPP-GMR elements such as those disclosed in, for example, JP 2003-298143A, JP 2005-086112A and JP 2006-261306A are capable of attaining a higher resistance and a greater resistance change amount, compared with a typical CPP-GMR element. However, a typical current-confined-path type CPP-GMR element has a problem as described below. In a typical current-confined-path type CPP-GMR element, the layer for producing the current confining effect is formed through an oxidation treatment, for example. JP 2003-298143A discloses a process of forming the first layer (the intermediate oxide layer) by subjecting a metal layer to an oxidation treatment. JP 2005-086112A discloses a process of forming the current control layer by subjecting a metal to an oxidation, nitriding or oxynitriding treatment. JP 2006-261306A discloses a process of forming a metal path of Cu in an AlCu oxide by forming an AlCu alloy on Cu and then performing an oxidation treatment. In such layers formed through an oxidation treatment, a great change in composition occurs during the process of formation, and the disorder of the crystal structure is thereby enhanced. Consequently, in MR elements including such layers, scattering of spin-polarized electrons occurs noticeably due to the disorder of the crystal structure, and a reduction in MR ratio is caused by spin relaxation.
Thus, it has been conventionally difficult to provide a CPP MR element having such a resistance that suppression of noise and suppression of the effect of spin torque are possible and capable of attaining a high MR ratio.